If the DS1270Y was powered up and then powered down, the “freshness” seal on the battery is opened and current will be drawn from the battery to maintain the contents of the SRAM during power down. The DS1270Y should be able to maintain at least 5 years of data retention in powered down mode once the freshness seal has been opened. This is the “Expected Data Retention Time” specification located on page 6 of the datasheet.
If the freshness seal is intact (ie, the part has never been powered up), ideally the battery will retain charge indefinitely as the leakage current when the freshness seal is intact is negligible.